Inorganic particles that are sensitive to atmospheric materials such as moisture can be coated using liquid-based or vapor phase methods. For example, binders and mineral or metallic colloids can be dispersed in liquid media and distributed over the surface of the inorganic particles to be coated. However, the coating materials must be insoluble in, and non-reactive with, the solvent used. Vapor phase coating methods, such as using fluidized bed reactors, require high temperatures and lengthy reaction times, which generally result in some surface reaction, or modification of the particles to be coated. Further, this type of equipment is complex and expensive.
Long persistence phosphors are known which, after having been struck by light from a light source, continue to emit light after the initial light source is extinguished. As an example, europium activated alkaline earth metal sulfides, such as SrS:Eu and CaS:Eu, are known that emit in the orange-red portions of the spectrum. Red-emitting phosphors having long persistence are very rare, but they are very useful, particularly for safety equipment, such as ‘EXIT’ signs and the like.
However, these phosphors are water sensitive, and they cannot be exposed to moisture without deleterious effects on their light emission. Thus they must be protected to prevent exposure to moisture.
Coating or encapsulating water sensitive phosphors with moisture insoluble materials has been tried. The coating particles can be mixed with the phosphor particles to be coated in a liquid medium, but both the coating material and the phosphor must be insoluble in, and non-reactive with, the liquid. In the case of phosphors, moisture sensitive phosphors cannot be immersed in solutions including any moisture.
Vapor phase coating has also been tried; for example, tetraethoxysilane or tetramethoxysilane can be passed through a fluidized bed including phosphor particles to deposit a silica layer, but this is an expensive process, and the high temperatures required can destroy the phosphor particles.
Thus it would be desirable to be able to apply a coating of an inorganic material onto inorganic particles to form a protective coating, or a coating that can modify particular surface properties, or impart a particular surface functionality to the inorganic particles.
In particular, a method of applying a water impervious coating onto phosphor particles that are sensitive to moisture, without the aid of an aqueous liquid medium, and a method that does not change the color or the strength of the phosphor emission, would be highly desirable.